Vacuum gap and vacuum switch devices including electron tunneling suppressing gas



Oct. 6, 1970 1'. A. DELCHAR 3,5325920 VACUUM GAP AND VACUUM SWITCH DEVICES INCLUDING ELECTRON v'I'UNNELING SUPPRESSING GAS Filed Oct. 10, 1968 2 Sheets-Sheet 1 FIG.

PUL 8 E SOURCE 'l/v vav TOR. TREVOR A. DELC HAR,

' HIS ATTORNEY Oct. 6, 1970 T. A. DELCHAR 3,

VACUUM GAP AND VACUUM SWITCH DEVICES INCLUDING ELECTRON TUNNELING SUPPRESSING .GAS Filed 001;. 10 1968 2 Sheets-Sheet 2 FIG. 2

' PULSE sou/m5 S g t 80 g 60 k I 00 ADM/TTED HERE if, AT l0" torn k k 20 q; \I v NO. OF PULSES V THE VOR A. vac/MR,

HIS ATTORNEY 3,532,920 VACUUM GAP AND VACUUM SWITCH DEVICES INCLUDING ELECTRON TUNNELING SUPPRES- SING GAS Trevor A. Delchar, Schenectady, N.., assrgnor to General Electric Company, a corporatlon of New York Filed Oct. 10, 1968, Ser. No. 766,545 Int. Cl. H01j 1/02, 1/38, 19/70 US. Cl. 313-107 9 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to vacuum are devices such as vacuum switches and triggerable vacuum gaps and, more particularly, relates to such devices which operate at relatively high currents and require that a high breakdown voltage be maintained over a number of arcing operations.

Vacuum are devices, such as those set forth in Lee et al. Pat. No. 2,975,255 and Latferty Pat. No. 3,087,092, which are examples of vacuum switches and triggerable vacuum gaps, respectively, have achieved a significant status in the arc interrupter field. One of the important reasons for the unique ability of the vacuum switch and triggerable vacuum gap to perform the arc interruption and switching functions which they perform, is the exceedingly high dielectric strength of vacuum which permits such devices to hold off very high voltages before breaking down at a predetermined time. The breakdown voltage of a vacuum arc device is, therefore, a criteria of the effectiveness of the device.

During the operation of vacuum arc devices, it very often appears that the breakdown voltage of a device degrades or decreases with arcing operations. A primary reason for such degradation of breakdown voltage is due largely to the fact that, once an arc has been struck between an arc-cathode and an arc-anode, the cathode spot erodes the arc-cathode so that pointed irregularities form thereupon. Subsequently these points, in the presence of high electric fields, are able to emit electrons by quantum-mechanical tunneling. When the voltage applied between the arc-electrodes reaches a certain critical value, the point at which electron tunneling occurs and/ or the anode opposite the same becomes sutficiently overheated that breakdown occurs.

In the prior art, efforts have been made to stabilize the breakdown voltage of arc-devices by pre-forming or pre-arcing the electrodes, so that a stabilized value can be achieved. Heretofore, efforts have not been made to change the nature of the surface to inhibit tunneling of electrons which lowers the breakdown voltage.

Accordingly, an object of the present invention is to provide vacuum are devices having high and reproducible breakdown voltages.

Still another object of the present invention is to provide means for treating the surface of vacuum arc-electrodes to inhibit low voltage breakdown thereof.

Still another object of the present invention is to provide vacuum arc devices having means for inhibiting the low breakdown characteristics of the primary arc United States Patent thereof and for maintaining such characteristic over long life-time operation.

Briefly stated, in accord with one embodiment of the present invention, I provide vacuum are devices including an hermetically-sealed envelope, evacuated to a pressure of less than 10- torr, and icluding therein a pair of primary arc-electrodes defining therebetween a primary breakdown gap. At least one of the electrodes is comprised of a high vapor-pressure material which may tend to form surface irregularities during arcing. To prevent the surface irregularities, so formed, from forming electron tunneling points and degrading the breakdown characteristic of the device, I provide means for causing a small but finite pressure of a gas which inhibits and suppresses electron tunneling from such points formed during arcing. Upon extinction of the high-current arc, the electron tunneling-suppressing gas condenses upon newly-formed surface irregularities and, by its presence thereat, prevents high field electron tunneling therefrom, thus maintaining a high-voltage breakdown characteristic for the device. In accord with yet another embodiment of the invention, I provide a reservoir of material capable of thermally evolving a continuing supply of the electron tunneling-suppressing gas during operation, so that the device may be operative at high-breakdown voltages over many arcing incidents.

The novel features believed characteristic of the present invention are set forth in the appended claims. The invention itself together with further objects and advantages thereof may best be understood with reference to the following detailed description taken in connection with the appended drawings in which FIG. 1 illustrates a triggerable vacuum gap constructed in accord with the present invention,

FIG. 2 is a vertical cross-sectional view of a vacuum switch constructed in accord with the present invention, and

FIGS; is a graph of the breakdown voltage as a function of breakdown incidence of a typical arc-electrode material as utilized in devices in accord with the present invention.

The triggerable vacuum gap of FIG. 1 comprises an insulating, gas-impervious envelope 1 which includes a first flanged disc member endwall assembly 2, a cylindrical sidewall member 3, and a second end closure mem her 4. A pair of primary arc-electrodes S and 6 are supported in spaced-apart relation within said envelope defining a primary gap 7 therebetween. Electrode 5 is generally a cylindrical member having an axial aperture therein. The aperture is tapered radially outwardly at the interior-depending portion thereof to provide a bore in the end of the electrode '5 having an exterior cylindrical portion 8 and an interior conical portion 9. Trigger assembly 10 is mounted within the aperture of electrode 5.

Electrode 5 is supported within the envelope 1 by endwall assembly 2 while electrode 5 is supported from end closure member 4 by means of electrode support member 11 which is hermetically sealed to the closure member 4 by welding, brazing, or other suitable techniques. Electrode 6 may be of any suitable size and configuration so as to maintain a discharge with electrode 5.

A metallic shield 12 having a generally cylindrical configuration with a ferruled open end 13, to prevent spurious arcing, is suspended from endwall assembly 2 and extends well past the gap 7 between electrodes 5 and 6. Shield 12 is utilized to preclude metal sputtered or evaporated from the arc-electrodes 5 and 6 from coat ing the inner surface of cylindrical sidewall member 3 of envelope 1 and thus degrading the insulating characteristics thereof.

Trigger assembly 10 comprises, for example, a cylindrical ceramic member 14 coated with a thin layer 15 of a substance which emits ionizable particles upon the application of a trigger pulse thereto. Typically such material may be an electrically-conductive, gas-charged material such as the hydride of titanium, hafnium, zirconium, or thorium, or the hydride of rare earth metals, for high temperature operation. Alternatively, the coating may be of a high vapor pressure metal as, for example, copper, beryllium, or aluminum. A groove 16, scored through layer 15 and into the ceramic of cylinder 14, separates layer 15 into a trigger-anode section and a trigger-cathode section. The position of groove 16 is chosen so that when trigger assembly is positioned within electrode 5, the junction between the bore 8 and the conical bore 9 is slightly below the lower edge of groove 16. A metallic cap 17 is suitably aflixed to the inner end of trigger assembly 10, so as to be in good electrical contact with layer 15. The upper portion of layer and end cap 17 comprises a trigger anode while the lower portion of layer 15 comprises a trigger cathode and is electrically in contact with primary arc-electrode 5. A wire 18, soldered or otherwise aflixed to cap 17 extends outwardly through the ceramic member 14 to provide means for applying a trigger pulse to the trigger anode. The trigger electrode assembly described herein is essentially as that of the aforementioned Lafferty patent. Alternatively, the trigger electrode may be any one of the electrodes described in the copending applications of J. M. Lafferty, Ser. Nos. 580,998, filed Sept. 21, 1966, now Pat. No. 3,465,192, and 704,935, filed Feb. 12, 1968, now Pat. No. 3,465,205, the disclosures of which are incorporated herein by reference thereto.

An hermetic seal is completed over the aperture in end wall assembly 2 by means of disc member 19, cylindrical ceramic member 20, and metallic disc 21. Hermetic seals are formed between each of these members and between disc 21 and wire 18. End wall assembly 2 is provided with a flange 22; end closure member 4 is provided With a flange 23, the flanges being adapted to form hermetic seals 24 with ceramic sidewall member 3 during the process of evacuating the device and electrode support member 11 is hermetically sealed to closure member 4.

Envelope member 3 and ceramic member 13 are fabricated from gas impervious, nonconducting material which may be hermetically sealed to a metal member. More specifically, it is important that these members, particularly, envelope member 3, be impervious to such gases as helium, which may, through long-term diffusion, destroy the vacuum necessary for such devices. Generally, any gas impervious ceramic may be utilized, such as for example, Coors V-200 or American Lava T-164. Alternatively, high-density aluminum oxide or other similar ceramic bodies may be used. It is to be understood, however, that although specific materials have been enumerated, any gas impervious ceramic or glass which may be hermetically sealed to metallic members may be utilized.

Arc-electrode 5 and arc-electrode 6, both of which are adapted to have surfaces suitable for sustaining the footprints of an electric are, are fabricated from a high vapor pressure material which is suitable to provide a ready supply of metallic ions which form conduction carriers for the high-current arc to prevent vapor starvation. Such materials are set foth, for example, in the United States patent to Lee et al. No. 2,975,256. Such materials, however, are susceptible of forming surface irregularities under the influence of a high-current arc. Certain of these materials, particularly, copper and nickel, are not only susceptible to such formation but are also ideally suited to have the voltage breakdown characteristics thereof improved in accord with the present invention.

In order to be operative within vacuum are devices in accord with the present invention, such high vapor pressure materials must be free of oxygen and other deleterious ionizable gases, in the bulk, to the point of 4 containing less than 10- parts thereof. Such materials may readily be prepared by multiple pass zone refining or by the special zone-refining process disclosed and claimed in US. Pat. No. 3,234,351Hebb. Electrode support member 11 should be a material which, although sufliciently electrically conductive as to provide little resistance for current flowing therethrough, should be large enough and be a sufliciently poor thermal conductor so as to prevent the major portion of the heating of the surface region of arc-electrode 6 from reaching endwall member 4. Such material may conveniently be tungsten or molybdenum, which are refractory and not particularly good thermal conductors. Because of the refractory nature of these materials, they need not be subjected to such zone-refining techniques for purification, but may merely be outgassed, as for example, for several hours at 2000 C. to remove any free oxygen therefrom.

In accord with the present invention, the surfaces of the high vapor pressure portions of arc-electrodes 5 and 6, which constitute the arcing faces, are impregnated with a thin adsorbed layer of a material which, in gaseous form during arcing, has a partial pressure within the device of approximately 10" to 10 torr. Upon extinction of the arc, this gaseous material migrates to and settles upon the newly-cleansed and eroded surfaces of the cathode electrode to so affect the surface characteristics thereof that the probability of electron tunneling in applied fields of the typical voltage hold-off magnitude, is greatly decreased. One such gas is carbon monoxide. When carbon monoxide, suflicient to provide an ambient pressure during arcing within the device in the range of 10- to 10 torr, is present upon the surface of the arc-electrodes in the quiescent state, such a quantity is of the order of one monolayer thick and has a weight of approximately 10 to 50 micro-micrograms per square centimeter of electrode surface.

Suflicient electron tunneling-suppressing gas to cover the arc-electrode surfaces, and provide the requisite ambient thereof during arcing conditions may be formed upon the surface of the arc-electrodes by admission of the gas after evacuation of the arc-device during formation. Thus, for example, after fabrication of the device and before final sealing, the device, including the electrodes, may be heated to a temperature of approximately 500 C. in an atmosphere of hydrogen at a pressure of approximately 5 torr for several hours. After pumping to 10- millimeters of mercury to remove the hydrogen, carbon monoxide at a pressure of approximately 10- to 10" torr is admitted. The final sealing is then effectuated.

Since the amount of the electron tunneling-suppressing gas, as for example carbon monoxide, is very small in quantity, the possibility exists that repeated arcings thereof may'cause this gas to clean-up and lose its effectiveness. Accordingly, in devices in which repeated arcings are contemplated, it is desirable that means he provided for causing a continuous supply thereof to be available. One such means is provided in the device of FIG. 1 by providing closure member 4 with an annular groove 25, shielded from the primary discharge by an overhanging annulus 26. A quantity of a material which, upon thermal decomposition, yields the desired electron tunneling-suppressing gas may be placed in various physical forms, herein illustrated generally by coils 27. Coils 27 are available for external heating by means of coils 28 which may be inserted into the cavity of closure member 4, but not within the hermetically sealed portion of the triggerable gap device, to raise the temperature of the reservoir material 27 to a temperature sufficient to decompose enough of it to cause a suitable pressure of the suppressing gas to be evolved. When the suppressing gas is carbon monoxide, molybdenum hexacarbonyl Mo (C0) is suitable, which decomposes at approximately C. to yield carbon monoxide and molybdenum. A

suflicient quantity of molybdenum hexacarbonyl may be provided so that only a minor amount of heating may be necessary to release a very small quantity to cause the desired pressure to be maintained.

Alternatively, the thermal conduction characteristics of arc-electrode support member 11 and of shield member 26 may be so chosen that a sufiicient quantity of heat from an electric are sustained by arc-electrodes 5 and 6 may reach the remote reservoir 27 to cause only suflicient heating for a limited amount of carbon monoxide to be released upon each arcing incident. Thus, each time an arc is stricken within the device, a new recharging amount of carbon monoxide is liberated so that any carbon monoxide cleaned up during arcing may be replaced. Appropriate thermal conductivity designs are well within the routine design criteria of the prior art.

In accord with another embodiment of the invention, a vacuum switch may incorporate the inventive features described hereinbefore. In FIG. 2, a vacuum switch having essentially the same configuration as the triggerable vacuum device of FIG. 1, and in which similar parts bear like reference numerals is shown in vertical cross section. The device of FIG. 2 includes an evacuable, hermetically-sealed envelope 1 including an end plate 2, a cylindrical sidewall member 3, an opposite end closure 4, and a pair of primary arc-electrodes 5 and 6 defining a primary breakdown gap 7. Poorly thermally conductive support member 11 of arc-electrode member 6 is interposed between closure member 4 and arcing surface portion 6. A shield member 12, having a ferruled end 13, is juxtaposed about arc-electrodes 5 and 6 to protect sidewall member 3. A bellows member 30 is connected between endwall member 2 and electrode support member 31 which, by reciprocable motion, may provide for contact between arc-electrodes 5 and 6.

When the device is used as a recloser and is in the normally closed position during circuit conducting operation, the separation of arc-electrode 5 from arc-electrode 6 results in the provision of charged particles within the gap to cause the establishment of a primary are between primary arc-electrodes 5 and 6. As in the device of FIG. 1, a reservoir 27 is provided which decomposes upon thermal energization to form an electron tunneling-suppressing gas of the order of to 10 torr ambient pressure within the device. As with the device of FIG. 1, this reservoir may be thermally activated by coil 28 or may be activated to cause partial decomposition by the thermal heating through conduction from the arc-electrode 6 during each arcing incident.

In devices in accord with the present invention, protuberances and pointed surface deformities are located primarily on the cathode electrode. Such cathode electrode surface irregularities are responsible for the degradation of the breakdown characteristics of prior art devices. Accordingly, if the device is to be operated upon direct current or if other means are provided for always causing a particular arc-electrode to be the arc-cathode when the triggerable vacuum gap is fired, or when the vacuum switch is caused to move from a nonarcing to an arcing condition, it is only necessary that the surface of one electrode be provided with a suflicient coating of the electron tunneling-suppressing gas to avoid degradation of the characteristics thereof. As a matter of fact, it is only necessary, in such instances, that the cathode electrode be comprised of an arcing surface having at least one high vapor pressure constituent thereof to provide a source of conduction carriers to prevent vapor starvation. In such instances, the remaining electrode may be of a refractory metal and, since refractory metals, such as tungsten and molybdenum, are not subject to the formation of gross surface irregularities such as are high vapor pressure metals such as copper and tin, they need not have the benefit of the present invention.

In most operations, however, triggerable vacuum devices and vacuum switches are not operated under such conditions. Generally, either electrode may be anode or cathode upon the establishment of a high-current arc therebetween. Accordingly, in such instances, both are electrodes should be comprised of a high vapor pressure material such as copper or nickel, both of which are subject to the formation of surface irregularities upon the establishment of a high-current arc therebetween; In this instace, both arc-electrodes must be protected with a coating of electron tunneling-suppressing gas sufiicient to pre vent the degradation of the breakdown characteristics thereof. This requires a surface concentration of approximately one monolayer and, in the instance of the utilization of carbon monoxide, the preferred electron tunnelingsuppressing gas in accord with the present invention, a concentration of approximately 10 to 50 micro-micrograms per square centimeter of electrode surface. Ideally, 40 micro-micrograms per square centimeter is desirable.

The operative characteristics of the present invention are such that the newly-formed surface irregularities are rendered uniquely clean and free of contaminants by the cleansing action of the cathode spot. Such clean surfaces are extremely attractive for adsorption of ambient gases. Since the prime ambient gas present in the operation of devices in accord with the present invention is the electron tunneling-suppressing gas, preferably, carbon monoxide, the carbon monoxide is rapidly attracted to the clean surfaces and adequately deposits itself thereover, completely covering the clean surface, including any pointed irregularities which otherwise serve as weak points at which electron tunneling may occur.

In general, the mechanism by which the invention operates is not clearly understood, but it is believed in some instances to be a complex function of the eifect upon the work function of the metal and the changing of the depth of the potential well which the adsorbed gas atom provides just outside the metal surface. With nickel electrodes it is known that the adsorption of carbon monoxide increases the work function of the nickel surface by approximately 1.1 electron volts. The mechanism with copper is not quite so well understood, but the results are clear and unequivocal.

Reference to FIG. 3 of the drawing shows the breakdown characteristics of one device utilizing copper areelectrodes constructed in accord with the present invention, wherein breakdown in kilovolts is plotted as ordinate and number of breakdowns is plotted as abscissa. Obviously, it may be seen that over a period of less than one hundred breakdowns, the breakdown voltage rose from something of the order of 45 kilovolts to nearly 60 kilovolts and stabilized at that voltage. When carbon mon oxide was added, the breakdown voltage rose from 60 kilovolts to stabilize at the order of kilovolts over approximately 400 breakdowns. Although the exact mechanism remains to be clearly identified, the results of the data shown by the curve of FIG. 3, clearly indicate the results of the utilization of electron tunneling-suppressing gases in accord with the present invention.

While the invention has been set forth herein with respect to certain preferred embodiments and specific examples thereof, many modifications and changes will read ily occur to those skilled in the art. Accordingly, I intend by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the present invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A vacuum arc device comprising:

(a) an hermetically-sealed envelope evacuated to a pressure of 10* torr or less and having at least a a portion thereof made of an insulator separating the envelope into two electrically-isolated parts;

(b) a pair of arc-electrodes disposed within said envelope and forming with one another a primary arcing s p,

(b said arc-electrodes each having an arcing 7 face adapted to sustain the footpoint of a high current electric arc,

(b at least one of said arc-electrodes in the vicinity of said arcing face comprising a major portion of a high vapor-pressure metal which forms surface irregularities under the influence of a high-current electric arc,

(b said one arc-electrode having at the arcing surface a chemisorbed layer of electron tunneling-suppressing gas; and

(0) means for initiating a high-current electric arc between said primary arc-electrodes.

2. The vacuum arc device of claim 1 wherein said high vapor-pressure metal is selected from the group consisting of copper and tin.

3. The vacuum switch of claim 1 wherein said high vapor-pressure metal is copper.

4. The vacuum arc device of claim 2 wherein said electron tunneling-suppresing gas is carbon monoxide.

5. The vacuum arc device of claim 2 wherein a reservoir of a compound which is thermally dissociable to provide said electron tunneling-suppressing gas is located within said envelope.

6. The vacuum arc device of claim 5 wherein said reservoir is adapted to be heated from without said sealed envelope to evolve said electron tunneling-suppressing gas.

7. The vacuum arc device of claim 5 wherein said reservoir is located within the limited influence of said primary arc-gap so as to evolve a limited amount of said electron tunneling-suppressing evolving gas upon each arcing thereof.

8. The vacuum arc device of claim 5 wherein said reservoir comprises a substance which is thermally dissociable to yield carbon monoxide.

9. The vacuum arc device of claim 8 wherein said substance is molybdenum hexacarbonyl.

References Cited UNITED STATES PATENTS 2,310,002 2/1943 Van Geelet a1. 313107 X 3,331,981 7/1967 Lafferty 313178 3,450,928 6/1969 Co bine 313311 JAMES W. LAWRENCE, Primary Examiner P. C. DEMEO, Assistant Exmainer US. Cl. X.R. 

